Radio frequency (RF) communication systems for the transmission of data information (i.e. binary coded information) are well-known in the art. RF data communication systems generally provide a single channel data rate to their users. In these systems, the modulation and error coding are designed to provide acceptable performance for users at the edge of the desired coverage area, where generally worst case signal quality conditions are experienced.
It is well-known that, at signal quality levels typical of those found in closer proximity to a transmitting antenna (rather than at the edge of a radio coverage area), higher data rates with corresponding higher data throughputs are possible. It is also well-known that a relatively wide dynamic range of signal quality levels (e.g. 20-80 dB or decibels) typically exists within the coverage area of a mobile radio communication system. Therefore, users of prior art data communication systems who experience signal quality levels significantly above those found near the fringe of the coverage area generally suffer a lower grade of performance, in terms of data throughput, than would otherwise be possible.
In the field of wireline telecommunications, data modems that provide multiple data rates in response to signal quality levels are well-known. The methods used in this art, however, are not well-suited for application to radio data systems in general, and particularly to radio systems employing Time Division Multiple Access (TDMA). In TDMA systems, the radio channel is divided into a series of time slots of predetermined constant duration, which are typically further grouped into frames, each frame containing a predetermined number of time slots. Multiple users are allowed to access the radio communication channel by transmitting in one or more time slots in each frame. Thus a complete communication is composed of a series of multiple transmissions, such that the duration of each transmission is equal to the time slot duration.
Radio data communication methods typically transmit data in variable length messages referred to as packets. Packets are formed by dividing the data into a series of fixed-size protocol units referred to as blocks. The combination of the data block size, the data transmission rate, and the TDMA slot size determines how effectively the TDMA channel can be used. For example, if an integer number of blocks would not fit evenly into each time slot, the capacity representing the fractional block may go unused, reducing the available throughput of the channel. Alternately, a synchronization method could be implemented to permit all of the data capacity to be utilized, but such techniques are often complex. This additional complexity manifests itself in increased cost and in additional communication overhead that also reduces available throughput.
Assuming a predetermined time slot duration, it is possible to choose a block size that avoids these problems for a single transmission data rate. A problem arises, however, when seeking to provide a channel that will support multiple data rates.
Accordingly, a need arises for providing a plurality of data rates for use with an RF data system so that users may select that data rate that provides the best performance for their signal quality level. It is further desired that the multiple data rates be provided in a manner such that a TDMA communication channel can be utilized efficiently by a packet data protocol.